1. Field of the Invention
This invention is related generally to safety valves. More particularly, this invention pertains to subsurface safety valves which employ a curved flapper for controlling fluid flow through a production tubing string.
Surface controlled, subsurface safety valves (SCSSVs) are commonly used to shut in oil and gas wells. Such SCSSVs are typically fitted into production tubing in a hydrocarbon producing well, and operate to block the flow of formation fluid upwardly through the production tubing should a failure or hazardous condition occur at the well surface.
SCSSVs are typically configured as rigidly connected to the production tubing (tubing retrievable), or may be installed and retrieved by wireline, without disturbing the production tubing (wireline retrievable). During normal production, the subsurface safety valve is maintained in an open position by the application of hydraulic fluid pressure transmitted to an actuating mechanism. The hydraulic pressure is commonly supplied to the SCSSV through a control line which resides within the annulus between the production tubing and a well casing. The SCSSV provides automatic shutoff of production flow in response to one or more well safety conditions that can be sensed and/or indicated at the surface. Examples of such conditions include a fire on the platform, a high/low flow line pressure condition, a high/low flow line temperature condition, and operator override. These and other conditions produce a loss of hydraulic pressure in the control line, thereby causing the flapper to close so as to block the flow of production fluids up the tubing.
2. Description of the Related Art
Most surface controlled subsurface safety valves are xe2x80x9cnormally closedxe2x80x9d valves. This means that the valves utilize a flapper type closure mechanism which is biased in its closed position. In many commercially available valve systems, the bias is overcome by longitudinal movement of a hydraulic actuator. In some cases the actuator of the SCSSV comprises a concentric annular piston Most commonly, the actuator comprises a small diameter rod piston located in a housing wall of the SCSSV.
During well production, the flapper is maintained in the open position by a flow tube connected downhole to the actuator. From a reservoir, a pump at the surface delivers regulated hydraulic fluid under pressure to the actuator through a control conduit, or control line. Hydraulic fluid is pumped into a variable volume pressure chamber (or cylinder) and acts against a seal area on the piston. The piston, in turn, acts against the flow tube to selectively open the flapper member in the valve. Any loss of hydraulic pressure in the control line causes the piston and actuated flow tube to retract, which causes the SCSSV to return to its normally closed position by a return means. The return means serves as the biasing member, and typically defines a powerful spring and/or gas charge. The flapper is then rotated about a hinge pin to the valve closed position by the return means, i.e., a torsion spring, and in response to upwardly flowing formation fluid.
In some wells, high fluid flow rates of as much as 250 million cubic feet or more per day of gas may be produced through the SCSSV. In high flow rate wells, it is well known that curved or arcuate flappers may be used to provide a larger inside diameter, or bore, in the SCSSV as compared to a flat flapper. Examples of such SCSSVs are described in U.S. Pat. Nos. 2,162,578; 4,531,587; 4,854,387; 4,926,945; 5,125,437; and 5,323,859. Curved flapper arrangements enable a larger production tubing inner diameter and, thus, allow for a greater rate of hydrocarbon production through the valve area.
In either flat or curved flappers, as the tubular piston and operator tube retract, the flapper closure passes across the lower end of the operator tube and throttles the flow as it rotates toward the closed or xe2x80x9cseatedxe2x80x9d position. At high flow rates, a high differential pressure may be developed across the flapper that may cause distortion and warping of the flapper as it rubs against the operator tube. Also, a flapper seat may be damaged if it is slammed open against the valve housing or slammed shut against the valve seat in response to the high-pressure differentials and production flow regimes. Damage to the flapper seat or leakage around the flapper may also occur if the flapper is closed on any debris in the well, such as sand or other aggregate that may be produced with the hydrocarbons.
In prior art SCSSVs, the flapper is seated in a variety of configurations. The flapper may be seated against an annular sealing face, either in metal-to-metal contact, or metal against an annular resilient seal.
In U.S. Pat. No. 3,955,623 discloses a flapper having a flat, annular sealing face. The flapper is engagable against a flat, annular valve seat ring, with sealing engagement being enhanced by an elastomeric seal ring that is mounted on the valve seat.
U.S. Pat. No. 4,457,376, the valve seat includes a downwardly facing, conical segment having a sloping sealing surface. The flapper closure member has a complimentary, sloping annular sealing surface that is adapted for surface-to-surface engagement against the conical valve seat surface.
U.S. Pat. No. 5,125,457, (expired) also presents a curved flapper. The flapper has a sealing surface with a convex spherical radius which seats in a matching concave housing. It also has a concave spherical portion constructed of an elastomeric material. The spherical radius flapper/seat has an alternate embodiment shown in U.S. Pat. No. 5,323,859. This patent teaches metal-to-metal sealing surfaces with no resilient seal.
In U.S. Pat. Nos. 5,682,921, and 5,918,858 a flat sealing surface is provided on both the flapper and the seat, fashioned in a sinusoidal undulating shape and having a combination metal and resilient seal.
In all these arrangements, the flapper rotates about a hinge assembly that comprises a hinge pin and a torsion spring. It will be appreciated by those of ordinary skill in the art, that structural distortion of the flapper, or damage to the hinge assembly which supports the flapper for rotational movement into engagement with the valve seat, can cause misalignment of the respective sealing surfaces, thereby producing a leakage path around the flapper.
Misalignment of the flapper relative to the valve seat may also be caused by the deposition of sand particles or other debris on the valve seat and/or sealing surfaces. Sand may be produced in both gas and oil wells, under low flow rate conditions as well as high flow rate conditions. It is particularly difficult to obtain positive sealing engagement of either flat or curved flappers and valve seats in low-pressure, sandy environments.
The integrity of the sealing engagement between the flapper and valve seat may be compromised under low flow rate conditions, while the same safety valve may provide positive closure and sealing engagement under high flow rate, high differential pressure conditions In this respect, slight misalignment may be overcome by high-pressure impact and engagement of the flapper against the valve seat. However, the same misalignment may produce a leakage path under low differential pressure conditions. Such misalignment will prevent correct seating and sealing of the flapper. The result is that a large amount of formation fluid may escape through the damaged valve, wasting valuable hydrocarbon resources, causing environmental pollution, and creating potentially hazardous conditions for well operations personnel. During situations involving damage to the wellhead, the well flow must be shut off completely before repairs can be made and production resumed. Even a small leak through the flapper safety valve in a gas well can cause catastrophic damage.
The following U.S. patents pertain to SCSSVs having flapper closure mechanisms and are hereby incorporated by reference: U.S. Pat. Nos. 3,788,595; 3,865,141; 3,955,623; 4,077,473; 4,160,484; 4,161,960; 4,287,954; 4,376,464; 4,449,587; 4,457,376; 4,531,587; 4,583,596; 4,605,070; 4,674,575; 4,854,387; 4,890,674; 4,926,945; 4,983,803; 4,986,358; 5,125,457; 5,137,090; 5,263,847; 5,323,859; 5,423,383; 5,285,851; 5,918,858; 5,682,921.
The present invention provides an improved flapper and seat for a surface controlled subsurface safety valve (SCSSV). The SCSSV of the present invention provides a curved flapper having a novel sealing surface for engaging a novel corresponding sealing surface in the seat. The sealing surface of the flapper is configured to contact the sealing surface of the seat along a sinusoidal sealing line, or seam, such that the reactive force from the seat is normal to the sinusoidal seating line. Thus, a more effective seal is achieved when the flapper pivots to its closed position. In operation, the novel SCSSV will safely and effectively shut in a well below the earth""s surface in the event of damage to the wellhead or flow line, or in the event of a malfunction of any surface equipment, with the shut-in being accomplished whether the well is operating in low flow or in high flow conditions.
The present invention also provides an improved surface-controlled, subsurface flapper safety valve in which the flapper closure mechanism and valve seat are tolerant of irregularities, such as obstructions or surface distortions caused by sand deposits or erosion of their respective sealing surfaces. The present invention also provides an improved flapper mechanism and seat in an SCSSV assembly having, in one embodiment, a flapper having a spherical sealing surface, and a corresponding metallic seat having a conical sealing surface. In one aspect, the sealing surface of the flapper has a convex spherical configuration relative to the seat. The sealing surface of the seat, in turn, has a concave conical shape relative to the flapper. In such an arrangement, the present invention provides an improved valve seat for an SCSSV adapted to provide a positive seal against a curved or arcuate flapper closure mechanism to overcome imperfect alignment or surface finish of its sealing surface relative to the safety valve seat.
The present invention also provides an improved flapper mechanism and seat in an SCSSV assembly having, in another embodiment, a flapper having a spherical sealing surface, and a corresponding metallic xe2x80x9chardxe2x80x9d seat having a conical sealing surface. Disposed concentrically within the hard seat is also a xe2x80x9csoftxe2x80x9d valve seat made of a yieldable material such as an elastomer (nitrile, neoprene, AFLAS(copyright), KALREZ(copyright)), a thermoplastic polymer (TEFLON(copyright), RYTON(copyright), or PEEK(copyright)), or a soft metal (lead, copper, zinc and brass). The soft seat defines a concave spherical or conical segment. The surfaces of the hard seat and the soft seat are configured to lie in sealable contact within the spherical radius that defines the sealing surface on the flapper. The surfaces are configured to provide a positive seal along a continuous interface seam between the conical hard seat, the (optional) resilient soft seat and the concave spherical sealing surface of the flapper.
According to the foregoing alternative arrangement, a convex spherical sealing segment of the flapper is received in nesting engagement against the surface of the soft seat, and against the conical sealing segment of the hard seat. The nesting arrangement allows for some misalignment of the flapper relative to the valve seat without interrupting surface-to-surface engagement therebetween. In this respect, the surface of the soft seat will tolerate a limited amount of angular misalignment of the flapper that might be caused by structural distortion of the closure or deflection of the hinge assembly, enabling a low-pressure seal. Line contact between the convex spherical segment of the flapper and the conical hard seat serves to realign the flapper as pressure increases. The hard seat also supplies sufficient structural rigidity to enable a pressure seal at high pressures. Positive sealing engagement between the flapper and the hard and soft seats is also obtained in sandy environments by engagement of the yieldable seat which conforms about surface irregularities which may be caused by surface deposits or surface erosion caused by the production of sandy fines.
It will be appreciated by one of ordinary skill in the art, that the foregoing net result of this interaction, is a flapper and seat system that performs in a sandy environment throughout any pressure range required in a hydrocarbon producing well for both tubing retrievable and wireline retrievable SCSSVs, and for both hydraulic or electrically actuated embodiments thereof.
As has been described in detail above, the present invention has been contemplated to overcome the deficiencies of the prior equalizing safety valves specifically by disclosing significant improvements to the flapper closure mechanism and the corresponding seat. The novel features of the invention are set forth with particularity in Detailed Description of Preferred Embodiments and The Claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.